But on the topic:
(2v2 perspective)
Heavy TDs: In my eyes these units are fine, they're a huge investment that only work against armor and making them take 3 shots to kill a medium tank just doesn't make any sense, they're never used against medium tanks anyways, apart from Comets and Pershing.

JU87 Loiter: Removing the chance to cause crits should be fine, it's not that strong in 2v2 atleast since there's almost always some sort of anti air to deal with it

FireFly: Theres no issue with this vehicle, it's low rate of fire makes it so that you can easily kill it with StuGs etc, poor turret traverse makes it easily flankable. It's also slow so it can be dealt with hand held AT quite easily

Howitzers: The 600mp price tag is quite harsh in most cases, forcing you to save usually for a long time while you can't really produce any new units, let alone reinforce your squads especially when you really need the howitzer to be able to do anything, a price tag of around 350/75 should do the trick, making it be a hefty investment but not being too expensive.

Pak43: No real need for a cost change in my eyes, especially for OKW since they're manpower rich anyways, but ostheer is another story. No counters? For USF maybe, pack howitzer is your best bet but brits can deal with it with a mortar pit unless the Pak43 is build really far back in which case you don't even need to deal with it most of the times. And it's a teamgame anyways, there should be a counter for it in your teammates arsenal

Aura Effects: None of the aura effects should work with your teammate(s), just creates annoyingly strong combinations that make no sense.

Mark target: Should have a reduced effectiveness for teammates, maybe reduce the effectiveness to 25-50% so it can still be used to support your teammates.

I'm still using the good old cons to fight against okw, triple cons in to a mortar usually allows you to keep enough ground to get to mid game without any major issues, adding 2 guards to the mix with maxims and at guns supporting should do the trick.

It's something you kinda have to get used to, the worst I've seen so far is me getting a rank 1500 teammate (I'm top 50 RT) against a top 20 arranged team. That match was pure suffering to play but that's the reality of random teams. The only thing you can do to prevent this is to play at hours on which more people are online